Stormwater Quality Design Manual (May 2007) - City of Sacramento ...

Runoff Treatment Control Measure Fact SheetConstructed Wetland BasinHow does a constructedwetland basin work?Permanent pools of water are located throughout the constructedwetland basin, within: the forebay (which allows settling out oflarger particles); an open water zone and a wetland zone withemergent vegetation (providing desired biodiversity); and theoutlet zone (from which water is discharged to the downstreamstorm drain system or receiving water). An area above thepermanent pool is designed to retain the stormwater qualitydesign volume (WQV). The retained water mixes with anddisplaces water from the permanent pool, which drains to theConstructed wetland basinsshould not be confused withwet detention basins (wetponds), which are presentedelsewhere in this manual.Constructed wetland basinsare shallower and featuremore vegetative coveragethan wet detention basins.downstream storm drain system or receiving water over the design drawdown period (48 hours forWQV). Much of the water discharged during and following a storm event is water displaced from thepermanent pool which has previously been treated by natural processes.Treatment of the runoff occurs through a variety of natural mechanisms that occur in the wetland,including sedimentation, filtration, adsorption, and biological uptake. The aquatic plants provide energydissipation and pollutant removal by enhancing sedimentation and providing biological uptake.Supplemental water or perennial baseflow is needed to maintain the permanent pool at all times.Planning and Siting Considerations• Integrate constructed wetland basins into open space, natural areas, and other planned landscapedareas when possible. Avoid placing features in open space and wetland preserves where futuremaintenance of the water quality facility will be restricted or prohibited.• Provide aesthetic security fencing if required by the permitting agency.Design CriteriaDesign criteria for constructed wetland basins are listed in Table CWB-1. Use the Design Data SummarySheet provided at the end of this fact sheet (Table CWB-4) to record design information for the permittingagency’s review.Design ProcedureStep 1 – Calculate Water Quality Volume (WQV)Using the Appendix E in this Design Manual, determine thetributary drainage area and stormwater quality design volume(WQV) for 48-hour drawdown.Use the Design Data SummarySheet (Table CWB-4) to recorddesign information for thepermitting agency’s review.Step 2 – Determine Basin Minimum Volume forPermanent PoolThe volume of the permanent wetland pool (V pp ) shall be not less than 75% of the WQV.V pp ≥ 0.75 x WQVStormwater Quality Design Manual for the Sacramento and South Placer RegionsCWB-2 May 2007